This invention relates to a wound dressing made with a self-supporting elastomeric thin film of perforated polymer to be placed in contact with a wound. The film not only has excellent "release" properties, but also is so soft and "giving" as to permit being removed from the wound with a minimum of pain or trauma. The wound-contact properties of the film are such that it may be left in contact with the wound longer than would ordinarily be practical with minimal maceration of the neoendotheliazing wound bed and surrounding wound area. The wounds may be burns, lacerations, or post surgical wounds, which accounts for the dressing being referred to as a surgical dressing.
The function of such a dressing is to protect the wound and absorb exudate while maintaining permeability to oxygen to facilitate healing and avoid maceration of the skin. Such protection is effected, in part, with an absorbent pad, permeable to air, secured to the perforated thin film which is placed with its lower surface in direct contact with the wound to minimize the growth of neoepithelium (sometimes less correctly referred to as eschar) into the absorbent pad to which the neoepithelium would adhere. Such a dressing is referred to as a "pad only dressing", and is generally held in place with a cotton gauze bandage which is a separate article in relation to the dressing.
As used herein, the term "dressing" always refers to an absorbent pad of material in combination with the thin polymeric film bonded to at least one surface of the absorbent pad, and preferably both the upper and lower surfaces of the pad. It will be understood that as used herein, the term "lower surface" refers to the surface to be placed in contact with the wound, and the upper surface is spaced apart from the lower surface by the thickness of the absorbent pad. Since typically, both the upper and lower surfaces are faced with the film, the wound dressing is referred to in the art as a "film-faced wound dressing", though only the surface to be placed in contact with the wound may be faced with the film.
In some instances, to expedite bandaging the dressing to the wound, a modified form of the dressing is used in which the upper surface of the wound dressing is secured to an adhesive surface of a backing sheet. Typically, this is a skin-conformable thin laminar web of woven or nonwoven fabric made from a normally liquid-impervious, resilient synthetic resinous material. This backing sheet, referred to as `backing`, is preferably perforated (to breathe) adhesive tape or sheet, to hold the dressing on that part of the body which is wounded. The lower surface of the absorbent pad is faced with a perforated thin film placed in contact with the wound. Relatively narrow strips of backed dressings, from about 2 cm to about 4 cm wide, with releasable protective strips are used as finger bandages, of the type illustrated in U.S. Pat. No. 3,043,301 to Plantinga et al; wider ones about 6 cm.times.6 cm are used on larger areas of the body. Because the backing serves as a bandage which is united with and not separate from the dressing, this combination, in such a modified form, is referred to as a "backed dressing".
`Pad only`, and `backed` dressings of this general type in the prior art, each of which dressings utilizes a thin film in direct contact with the wound, are highly desirable. The pain of having such a dressing removed while the wound is healing is relatively remarkably less than with dressings which do not rely on a film (in contact with the wound) with specific "easy-off" properties for `release` from the wound's surface. The term "film" is used herein to refer to the thin self-supporting elastomeric film which, in my invention, is to be placed in direct contact with the wound.
The film is typically adhesively secured to an absorbent pad, made from conventionally used non-toxic woven or non-woven fabric, optionally coated with a coating, or containing a layer, of a flexible hydrophilic material adapted to absorb exudate. The pad is, of course, the main component of such a wound-dressing. The more desirable dressings not only absorb exudate effectively, but breathe well to accelerate healing of the wound, and are easily removed without inflicting pain. As will readily be appreciated, the adhesive layer bonding the pad to the film must meet numerous requirements relating to toxicity, disintegration when wetted with exudate, and of which requirements contribute to the cost of adhesive, which cost, coupled with the cost of applying the adhesive, add up to a substantial portion of the cost of manufacturing a wound dressing of this type. It is desirable to minimize this cost.
Smooth perforated film is used in the art to provide good release from the healing wound, thus result in minimal tearing of the eschar or scab. This in turn causes less pain than that caused by removal of a dressing which does not have excellent release characteristics. But a film, such as Mylar polyester film with excellent release characteristics may have a highly undesirable cutting action during removal of the dressing.
However, even among films with acceptable release characteristics one cannot predict at which end of the scale of acceptability its release might be; nor can one predict if the film has other characteristics which mitigate against its use, for example degradation due to contact with exudate, or due to exposure to heat and light, and most of all whether the perforated film will be soft enough not to have a cutting action on a healing wound. Moreover, an imperforate soft film, whether plastic or elastomeric, may provide good release, but its good release is masked though not negated, When the -film is foraminous, that is, multiply perforated (referred to herein as "perforated" for brevity) and the multiplicity of perforations have jagged edges. In the prior art, the goal was to maximize the open area to minimize adhesion of the film to the neoepithelium, while ignoring the adverse effect of the number of perforations on the cutting action of the film. The patient simply suffered the trauma of removal of the wound dressing, and tried not to move until the dressing had to be changed.
We have now realized that, even with a film having excellent release, the goal must be to minimize the open area (due to perforations) to minimize the growth of neoepithelium into the perforations, yet not adversely affecting the required "breathability", and water or moisture vapor transmission rate (MVTR) of the film.
Recently there has become available a perforate, thin, soft elastomeric polyurethane film, which provides more than about 5% of open area relative to the overall area of film, without tearing or unacceptably distorting the peripheral edges of the perforations. If the open area is more than about 5%, but is limited to 25%, I have found that a soft elastomeric film from a variety of polymeric materials exhibit a comparable combination of desirable strength, good release characteristics, lack of cutting action, and breathability. This has made it possible to experiment with a wide variety of perforated soft elastomeric self-supporting polymeric films to find those with properties best suited for a film-facing on a wound dressing.
Known perforated thin films currently in use in such film-faced dressings are flexible and thermoplastic, but they are not elastomeric, and are therefore referred to as being "plastic" as opposed to "elastomeric". Typical synthetic resinous materials suggested in the prior art for such films have been polyethylene terephthalate, sold under the Mylar brand, polyethylene, poly(vinyl chloride) ("PVC"), cellulose acetate, poly(vinylidene chloride) and nylon. Their choice in wound dressings was dictated less by their release properties than their susceptibility to being easily perforated, referred to as "punchability" because the convenient and conventional method of perforating a thin plastic film is to punch through, or die-cut it with a mechanical punching means. As will be apparent, their punchability is attributable in large part to the crystallinity of the resin, and even PVC which is regarded as being amorphous, or having low crystallinity relative to the other resins, when punched, results in a film having holes with peripheries which can cut, whether the peripheries are well defined or not.
Most of all, it was never suggested that the cost of securing the film to the absorbent pad may be minimized by eliminating the adhesive, and that this could be accomplished by selecting a film of a polymer which is thermally bondable to the pad without being degraded. But before this problem is attacked, one must find a soft film which can be acceptably perforated, since it must be perforated.
It is common experience that an edge of a sheet of paper can inflict a cut on whole skin. One can thus surmise that the edges of perforations in a plastic sheet, whether a hard, crystalline plastic such as polyethylene terephthalate, or a softer amorphous plastic such as poly(vinyl chloride), can be decidedly traumatic on a highly sensitive wound which is healing. In other words, there is insufficient "give" at the edge of a hole in a plastic sheet because prior art films lack the necessary elongation. This lack of "give" results in the periphery of a hole behaving like a knife-edge which produces a cutting action on tissue in contact with the periphery when the film is being removed. Even when there is only slight normal angular movement of the skin of the wounded person relative to the plane of the dressing, a lack of "give" and "softness" tends to produce some relative displacement between the film and the healing tissue, causing, at the very least, irritation, and often, considerable pain and trauma.
Films are perforated because an imperforate film, unless designed to have a water vapor transmission rate (WVTR) within a specified range, will generally provide neither adequate ventilation of the wound bed, nor passage of body fluids to the absorbent pad. Thus, it is essential for a film-faced wound dressing, to have essentially no restriction of fluid flow from the wound bed into the absorbent pad. To obtain this, the film is perforated with a multiplicity of closely spaced perforations, and irrespective of how good the release properties of a smooth, flexible plastic film such as has been used in the prior art for its good release properties, the sharp peripheries of holes in the film are highly objectionable because of the pain they inflict. By "sharp" I refer only to suitability for cutting and the ability to cut healing tissue, and not to peripheries which are so well-defined and free of jagged edges as to merit the designation "sharp".
As disclosed in the aforementioned '301 patent to Plantinga et al, there are limitations imposed on the perforations by the healing process. The size of the holes in the film must be quite small to prevent the neoespithelium or congealed exudate from adhering to the absorbent pad in the immediate vicinity of the holes. Yet, if too small, the holes are plugged with neoepithelium and decrease the effectiveness of the absorbent pad, the smaller the holes, the worse the plugging. Too many holes in prior art plastic film vitiate its physical integrity, and too few holes will interfere with the ventilation the film must provide. Even if one wanted a large open area with a very large number of holes per unit area, which may not be desirable, the holes in elastomeric films are not prone to being plugged, nor is the elastomeric film's integrity significantly impaired with a much larger ratio of open area to overall film area, compared with the ratio of open area to overall film area in plastic film.
The problem of patient comfort with sharp peripheral edges in punched film, and the inherently deleterious effect of the mechanical punching process on the tear resistance of the punched film have been well recognized. In U.S. Pat. No. 3,073,303, the film was thermally perforated so that portions of the film immediately adjacent and surrounding each of the perforations in the film are thicker than the remaining film. But the resulting film is too irregular and the raised peripheral edges contribute to severe discomfort in part attributable to the geometry of raised peripheries and in larger part attributable to the choice of polymers from which the film was made. Such polymers suggested for use included vinylidene chloride-vinyl chloride optionally plasticized, such as Saran 54L; polyethylene terephthalate such as Mylar; and crystalline polyethylene, because they are far more susceptible to being perforated thermally than elastomeric materials. My invention derives from discarding the use of raised peripheral edges, instead using a smooth, soft film which can be thermally bonded to an absorbent pad, and changing from a plastic film to an elastomeric one. By "smooth" I refer to the surface of the film which is essentially free of raised peripheral edges or crenelations. It was never heretofore realized that an elastomeric film can be so soft, and yet provide such good release with a concomitantly high comfort factor; nor that such a film, if satisfactorily perforated, would overcome the problems of the prior art films in film-faced wound dressings. Until the availability of perforated polyether polyurethane (PPU) film, this was not feasible. Such an elastomeric thin film has now been provided with uniformly spaced apart, uniform perforations which do not have jagged, crenelated, or built-up peripheries.